Hi Richard ,
The RadTabs do not have a client ID and that is why your approach does not work. In such case, you can either add an attribute on the server or create the tooltips on the client. Both the approaches are demonstrated in the demos below:
http://demos.telerik.com/aspnet-ajax/tooltip/examples/tooltiptreeview/defaultcs.aspx
http://demos.telerik.com/aspnet-ajax/tooltip/examples/tooltipcalendar/defaultcs.aspx
For your convenience I prepared a sample demo of the first approach and attached it to the thread. Let me know how it goes.
Best wishes,
Svetlina
the Telerik team